The present invention relates to a spiral plane bit. More particularly, the invention relates to a spiral plane bit which can be mounted and fastened on the outer surface of a rotary shank for use in a rotary cutter of a planer or the like.
A conventional rotary cutter, for example the type which is mounted on a planer for cutting the surface of wood material, is constructed in such a way that the planing bit is fastened to the outer surface of a rotary shank. In the past, two general categories of such cutters have been available. In one type, a straight planing bit is inserted and secured in a straight groove extending parallel to the axis of the rotary shank, while in the other a spiral planing bit is inserted and fastened in a spiral groove formed on the outer surface of the rotary shank.
It is pointed out that the former rotary cutter, that is the straight type rotary cutter, has the drawback that it generates high noise during cutting operation because the planing bit makes contact with a workpiece over the whole length thereof at the same time and it tends to form an inverted grain on the surface of wood material. To eliminate this drawback, it is necessary to mount a supplementary cutter. The latter rotary cutter, that is spiral type rotary cutter, was originally developed in order to eliminate the drawbacks inherent in the conventional straight type rotary cutter. In a spiral plane cutter which has a length equal to one pitch of a spiral line, for instance, the plane bit comes in contact with the workpiece at just one point at all times, which generates less noise and consumes less power for the cutting operation. On the other hand, there is a drawback with the spiral plane bit in that uniform bit edge protrusion can only be achieved with much difficulty. The aforesaid "bit edge protrusion" means the protrusion of the spiral bit edge in a peripheral direction from the surface of the spiral groove in which the spiral bit edge is to be received. In case of a spiral plane bit with fixed pitch length, the bit is oriented in opposite directions at points half a pitch length apart. Hence, a bit edge protrusion in the peripheral direction on one side results in a bit edge retraction on the other side at a point half a pitch length away. This means that it is substantially impossible to provide a uniform bit edge protrusion over the whole length of the spiral plane bit and this is a critical drawback with the conventional spiral type rotary cutter. It has been found that the above-mentioned drawback with the hitherto known spiral plane bits results from the geometrical shape and structure of the plane bit itself.